Spacecraft protected by a coating including pyroelectric/ferroelectric particles, and the coating material

ABSTRACT

A spacecraft protected by a coating includes a spacecraft such as a communications satellite has an external surface and a coating on the external surface of the spacecraft. The coating is formed of a binder, and a plurality of pyroelectric/ferroelectric pigment particles, preferably ferroelectric particles, bound together by the binder. Each ferroelectric pigment particle includes a ferroelectric pigment material having a ferroelectric/paraelectric transition. The coating may also include active secondary particles and/or inert particles. The coating may operate in a passive mode or an active mode with the application of a bias voltage to the coating.

This application is a continuation of co ending application Ser. No.10/051,672, now U.S. Pat. No. 6,478,259, filed Jan. 16, 2002, for whichpriority is claimed and whose disclosure is incorporated by reference inits entirety, which in turn is a continuation of application Ser. No.09/492,723, filed Jan. 27, 2000, now U.S. Pat. No. 6,405,979, for whichpriority is claimed and whose disclosure is incorporated by reference inits entirety.

BACKGROUND OF THE INVENTION

This invention relates to passive thermal control of spacecraft, and,more particularly, to spacecraft protected by an external coating whichaids in thermal control and protects the spacecraft against damage by aflux originating externally.

Spacecraft are subjected to a wide range of thermal environments duringservice. One side of the spacecraft may face away from the sun into thevoid of free space, while the other side faces the sun. Heat is radiatedinto free space from the side of the spacecraft facing away from the sunto cool the spacecraft, but the side of the spacecraft facing the sun isheated intensively by direct sunlight.

Active and passive temperature control techniques are used to maintainthe interior temperature of the spacecraft, which contains persons,electronic devices, and/or sensitive instruments, within acceptableoperating limits. Active temperature control usually involves mechanicalor electrical devices, such as heat pipes or electrical heaters. Thepresent invention deals with an approach that incorporates a basicpassive temperature control technique, but which may be used in anactive control mode as well.

One approach to passive temperature control uses surface coatings,sometimes termed “paints”, on the external surface of the spacecraft. Awhite coating, for example, has a low solar absorptance, while a blackcoating has a high solar absorptance. The selective application of suchcoatings to various elements of the spacecraft exterior greatly aids incontrolling their temperatures. The present invention deals with acoating that is useful in spacecraft temperature control applications.

In most cases, the coating desirably also provides electrical protectionto the spacecraft, in addition to providing passive thermal control. Aspacecraft is sometimes subjected to electronic charging induced by aflux of electrons originating from an external source. In one example ofextreme charging, a solar storm may eject a high flux of electrons fromthe sun. When the electron flux reaches the spacecraft, it subjects thesurface of the spacecraft to a large flux of electrons. These electronscan accumulate as a static charge and eventually produce arcing (i.e., adielectric breakdown and electrostatic discharge) at the surface of thespacecraft, which may structurally damage the spacecraft and/orinterfere with sensitive electronic equipment on or in the spacecraft.

Several passive coating-based approaches are known to protect spacecraftfrom this type of electrical damage. In one approach a multilayercoating is provided, wherein a top coating serves the thermal controlfunction and an underlying layer is electrically conductive to dissipateelectrical charge. Such multilayer coatings are heavy and are difficultto apply because the layers must be quite precisely deposited.Single-layer electrostatic-dissipative paints are also known forspacecraft use. One such white paint, based upon the aluminum-doped zincoxide pigment of the type disclosed in U.S. Pat. No. 5,094,693,typically has a solar absorptance of from about 0.18 to about 0.22. Thewhite paint described in U.S. Pat. No. 5,820,669 improves upon thisperformance by providing a solar absorptance of less than 0.1. Thesepaints provide excellent performance in a number of applications.However, the paints described in the '693 patent and the '669 patent arelimited as to the maximum flux of electrons that may be dissipatedbecause of the maximum electrical conductivities that are possible withtheir formulations, and therefore cannot perform some missions.

There is a need for a further improved thermal-control coating that isoperable and stable in a space environment, which has tailorable thermalproperties, and which protects the spacecraft against damage byexternally induced electronic fluxes of high magnitudes. The presentinvention fulfills this need, and further provides related advantages.

SUMMARY OF THE INVENTION

The present invention provides a spacecraft protected by a coating, andthe coating material. The color of the coating may be selected toprovide desired thermal properties. The coating is structured to protectthe spacecraft against electrical damage produced by the accumulation ofelectrical charge induced by intensive external fluxes of electrons. Asa result of its high index of refraction and resulting excellent hidingpower, the coating may be applied in thinner coatings than conventionalprotective coatings, reducing the weight and cost of the spacecraft. Thecoating pigment of the invention may be mixed with other pigments tooptimize performance for a wide variety of conditions. The coating maybe used in either a passive or an active mode.

In accordance with the invention, a spacecraft protected by apyroelectric/ferroelectric coating comprises a spacecraft having anexternal surface, and a coating on the external surface of thespacecraft. The coating includes a binder, and a plurality ofpyroelectric/ferroelectric pigment particles bound together by thebinder. The pyroelectric/ferroelectric pigment particles are preferablyferroelectric pigment particles, each of which comprises a ferroelectricpigment material having a ferroelectric/paraelectric transition.

In another embodiment, each pyroelectric/ferroelectric pigment particlemay be described as comprising a ferroelectric pigment material having adielectric permittivity exceeding about 200, typically from about 200 toabout 25,000, and an electronic band gap exceeding about 2.5 electronvolts. In yet another embodiment, each pyroelectric/ferroelectricpigment particle may be described as comprising a ferroelectric pigmentmaterial which stores electronic charge on the particle surface whenexposed to an electron flux, and which thereafter releases the storedelectronic charge over a period of time.

The present invention is a complete departure from the approaches of theprior art to multi-layer and single-layer paints and coatings whichprotect spacecraft from electronic charge accumulations. Only a singlelayer is used, avoiding the application difficulties experienced withmulti-layer paints. Previously, the single-layer paints had beendescribed as electrostatically dissipative (ESD), and the designapproach was based upon obtaining a sufficiently high electricalconductivity of the paint while retaining the desired thermalproperties. The high electrical conductivity dissipates the electroniccharge as it builds up, and eventually conducts the charge to ground.The success of this design approach in protecting the spacecraft isbased upon achieving a sufficiently high electrical conductivity in thepaint. This approach works well for many spacecraft applications, but islimited in its ability to protect the spacecraft against very highelectronic fluxes because there are physical limits on the ability toincrease the electrical conductivity of the paint while retainingdesired thermal properties.

In the present approach, the accumulation of charge on the surface ofthe particles and the coating is acceptable, as long as thataccumulation of charge does not produce a high surface voltage thatcould lead to arcing (that is, a dielectric breakdown and electrostaticdischarge) or other type of electrical damage. The coating absorbs andstores the electronic charge at the surface of the coating as itaccumulates during a high-flux event while preventing a significantincrease in surface voltage, and both simultaneously and thereaftergradually conducts the accumulated charge to ground. The coating isthereby “reset” for the next high-flux event. The coating effectivelyacts as an intentionally leaky thin capacitor applied over a large areaof the external surface of the spacecraft. The coating therefore has asmall electrical conductivity, expressed as a surface resistivity ofless than or equal to about 10¹⁰ ohms per square at room temperature.This conductivity is achieved by doping the pigment material to a levelthat its conductivity is sufficient to provide the required conductivityto the coating. Desirably, the electrical surface resistivity is notless than about 10⁸ ohms per square, inasmuch as the doping required toachieve such low electrical surface resistivity would likely adverselyaffect the optical properties of the coating.

When a conventional ESD paint is operated on a surface at a lowtemperature, the electrical surface resistivity of the pigment andthence the paint increases so that a portion of its electricalprotective capability is lost. In the coating of the present invention,the electrical surface resistivity of the pigment may change, but thepolarization capability of the coating stores the electrical charge topermit a gradual dissipation according to the electrical conductivitythat remains. The stored charge does not result in a high surfacevoltage that leads to arcing, because of the high dielectric constant ofthe pigment and thence the coating. The present coating thus providesprotection against electron fluxes that persists at very lowtemperatures, an important improvement over available ESD paints.

Materials which exhibit a ferroelectric/paraelectric transition, termedherein and in the art a “ferroelectric” material, are preferred for useas the pigment in such a coating. Such ferroelectric pigment materialsaccumulate electronic charge without a substantial increase in surfacevoltage, because they preferably have a dielectric permittivityexceeding about 200. The ferroelectric pigment materials are doped sothat they have sufficient electrical surface conductivity to conductaway the accumulated charge relatively slowly during and after theincidence of the external flux. They preferably have an electronic bandgap exceeding about 2.5 electron volts. Ferroelectric pigment materialshaving the ferroelectric/paraelectric properties may be manufactured invarious colors, which may be selected to produce the required thermalproperties in the coating. The whiter the ferroelectric pigmentmaterial, the lower its solar absorptance.

Stated alternatively, the present approach provides for an accumulationof electrical charge at the surfaces of the ferroelectric pigmentparticles during a high-flux event, while gradually conducting thecharge away both during the high-flux event and after it has ended. Thecoating is thus “adaptive” to the conditions experienced in space. Instandard low-flux conditions, any introduced electronic charge isconducted away substantially as it develops. As the coating is subjectedto fluxes of increasing electronic flux density, the coating harmlesslystores the charge in excess of that which may be conducted awayimmediately, and conducts that charge away after the external flux ends.Consequently, there is a recovery time which increases with increasingmagnitude and duration of the flux, but the coating of the presentinvention can protect against higher fluxes than possible with theconventional approach. The conventional approach requires that theelectrical charge produced by a high-flux event be conducted away as itis created in order to prevent a high voltage at the surface of thecoating. However, physical limits on the doping of conventional pigmentsprevent such conventional paints from dissipating electrical chargesufficiently rapidly in some high-flux conditions, with the result thatthere may be excessively high voltages at the surface of the paint andconsequent electrical arcs.

The present approach thus provides a coating which protects the surfaceof a spacecraft against excessive electrical charging over a wide rangeof conditions of external electron fluxes, while also providing passivethermal control with the possibility of active thermal control. Otherfeatures and advantages of the present invention will be apparent fromthe following more detailed description of the presently preferredembodiment, taken in conjunction with the accompanying drawings, whichillustrate, by way of example, the principles of the invention. Thescope of the invention is not, however, limited to this presentlypreferred embodiment.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic elevational view of a coating layer according tothe invention;

FIG. 2 is a schematic elevational view of the coating layer of FIG. 1,as applied to a substrate;

FIG. 3 is an equivalent circuit diagram for the coating layer of theinvention;

FIG. 4 is an idealized graphical depiction of the permittivity of aferroelectric/paraelectric coating pigment as a function of temperature;

FIG. 5 is an idealized graphical depiction of the polarization of aferroelectric coating pigment operating below the Curie temperature as afunction of applied electric field;

FIG. 6 is an idealized graphical depiction of the polarization of aferroelectric coating pigment operating at a temperature just above theCurie temperature and the polarization of a non-ferroelectric ESD paintpigment of the prior art, as a function of applied electric field;

FIG. 7 is a block diagram of a method for the preparation of a coatingaccording to the invention and the coating of a substrate;

FIG. 8 is a perspective view of a spacecraft having a coating layeraccording to the invention; and

FIG. 9 is a schematic drawing of an active control mode application ofthe coating layer.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 illustrates a coating layer 20 prepared according to theinvention. The paint layer 20 comprises pigment particles 22 mixed witha binder 24. The pigment particles include pyroelectric/ferroelectricpigment particles 22 a according to the present invention, and,optionally, active secondary particles 22 b and/or inert secondaryparticles 22 c. For example, the nonpyroelectric/ferroelectric pigmentparticles (serving as active secondary particles 22 b) disclosed in U.S.Pat. No. 5,820,669 may be mixed with the pyroelectric/ferroelectricpigment particles 22 a of the present invention. Prior to drying, acoating vehicle is also present, but the coating vehicle is evaporatedduring the drying operation. The compositions of the particles, thebinder, and the coating vehicle, their proportions, and the preparationof the coating layer will be described in greater detail subsequently.

FIG. 2 illustrates the coating layer 20 applied to an external surface25 of a substrate 26. The substrate 26 is typically the skin of thespacecraft and is at least somewhat electrically conductive.

The nature of the pyroelectric/ferroelectric particles 22 a may beunderstood by reference to their crystallographic structures. Allcrystals may be placed into one of 32 symmetry point groups as presentlyknown by crystallographers. These 32 point groups are subdivisions ofthe well-known seven basic crystal systems: cubic, hexagonal,rhombohedral, tetragonal, orthorhombic, monoclinic, and triclinic.Twenty-one of the 32 groups are noncentrosymmetric, in that the pointgroup lacks a center of symmetry. A lack of a center of symmetry isnecessary for the crystal to exhibit the phenomenon of piezoelectricity,whereby a homogeneous stress upon the crystal produces a net movement ofpositive and negative ions with respect to each other, resulting in anelectric dipole moment and thus polarization. Twenty of these 21noncentrosymmetric point groups exhibit piezoelectricity. Of the 20point groups which exhibit piezoelectricity, 10 are known to bepyroelectric (sometimes called polar). A pyroelectric crystal has theadditional characteristic of becoming spontaneously polarized andforming permanent dipole moments within a given temperature range.Non-ferroelectric pigment materials which lie within the 10 pyroelectricpoint groups are operable with this invention, but are not preferredbecause their polarization effects are relatively small. The 10pyroelectric (or polar) point groups are (in Schoenflies notation): C₁,C₂, C_(s) or C_(1h), C_(2v), C₄, D₄, C₃, C_(3v), C₆, and C_(6v). Anincluded (noncentrosymmetric point group) material class is theanti-ferroelectrics, which are polar at the level of thecrystallographic unit cell but do not have an apparent macroscopicpolarization.

A special subgroup of the 10 pyroelectric point groups is known asferroelectrics, which, with the appropriate doping, are the preferredmaterials for use as the pigment in the present invention. Theferroelectric material is distinguishable from the pyroelectric materialin that the polarization is reversible by an electrical field ofmagnitude of less than the dielectric breakdown strength of the crystalitself, a condition which is not present in a material designated asexhibiting pyroelectricity alone. The preferred ferroelectric materialsare thus characterized by both a spontaneous polarization resulting inpermanent dipoles within a given temperature range and thecharacteristic of the ability to reorient the polarization by anexternally applied electric field.

The particles 22 a are therefore described as“pyroelectric/ferroelectric”, a term used herein to mean that they are apyroelectric material but are preferably within the subgroup of theferroelectric materials. Because the ferroelectric materials representthe preferred embodiment, the following discussion will focus onferroelectric materials with the understanding that materials which arepyroelectric but not ferroelectric may also be used.

There are many ferroelectric crystals and ceramic solid solutions whichare operable within the scope of this invention. Of the ferroelectricsubgroup of the 10 pyroelectric point groups, several are of particularimportance. These include the tungsten bronze structure (for example,PbNb₂O₆), the oxygen octahedral structure (generalized by example ABO₃),the pyrochlore structure (for example, Cd₂Nb₂O₇), and layer structures(for example, Bi₄Ti₃O₁₂). Of further importance within the oxygenoctahedral structures are the ceramic perovskites which are ofparticular importance to this invention. The perovskites include, by wayof example, barium titanate BaTiO₃ along with its various solidsolutions such as BST (barium strontium titanates), PZT (lead zirconatetitanates), PLZT (lead lanthanum zirconate titanates), PT (leadtitanates), PMN (lead magnesium niobates), and sodium-potassium niobates(Na,K)NbO₃. Other operable ferroelectrics include PZN (lead zincniobates), PSZT (lead stannate zirconate titanates), PZ (leadzirconates), and SBT (strontium bismuth titanates). These materials aregiven only by way of example to define the scope of the operablematerials using crystal structure, and the invention is not limited tothese materials.

The pyroelectric/ferroelectric pigment is doped so that the coating,formed of pigment particles mixed into the binder, preferably has anelectrical surface resistivity of less than or equal to about 10¹⁰ ohmsper square at room temperature. If the surface resistivity of thecoating is greater than about 10¹⁰ ohms per square at room temperature,the coating may still be operable but the rate of leakage of the storedcharge will be so low that the coating may not be practical for mostapplications. Examples of operable dopants include lanthanum, niobium,gallium, gadolinium, and yttrium, and mixtures thereof, typicallyfurnished in the form of their oxides. The greater the volume fractionof the pigment particles in the coating, the lower may be theconductivity of the doped pigment material to be operable, so that theproperties are best expressed in terms of the coating surfaceresistivity rather than the pigment material.

The ferroelectric pigment particles 22 a comprise a ferroelectricpigment material having a ferroelectric/paraelectric transition. Thenature of such ferroelectric pigment materials will be discussed in moredetail subsequently, but generally they exhibit a ferroelectric state atlower temperatures, a paraelectric state at higher temperatures, and atransition therebetween. FIG. 3 illustrates, as an equivalent circuit,the electrical performance of the coating layer 20 having suchferroelectric pigment particles 22 a. An external flux of electrons,represented as a voltage source 28, is directed against an outer surface30 of the coating layer 20 remote from the substrate 26. Externalelectron fluxes in a space environment, such as produced by a solarstorm or by man-made sources, are typically intense but of relativelyshort duration and spaced apart temporally. The coating layer 20 acts inthe manner of a leaky capacitor, with an effective parallel capacitanceC_(p) and an effective parallel resistance R_(p). The opposite side ofthe coating layer 20 is in electrical contact with the substrate 26,which typically is held at a ground potential. When the externalelectron flux is applied to the coating layer 20, charge is accumulatedand temporarily stored at the surface of the capacitive coating layer20, and is gradually conducted to the substrate 26. Charge conductionoccurs simultaneously with the period of the external electron flux.However, any excess charge stored in the coating layer 20, above thatwhich may be conducted to ground while the external electron flux isunderway, is conducted to the substrate 26 after the external electronflux has ceased.

Significantly, the voltage V_(S) at the outer surface 30 remains lowbecause of the high permittivity of the ferroelectric/paraelectricmaterial. This voltage V_(S) is the key to preventing arcing and otherundesirable electrical occurrences at the outer surface 30. The voltageV_(S) must be maintained sufficiently low to prevent arcing and otherundesirable electrical occurrences. The stored charge may accumulate tovery high levels at the surface of the ferroelectric/paraelectricmaterial, as long as V_(S) remains low.

This protective concept is a departure from the prior design basis forelectrostatic dissipative (ESD) paints, where the material wasnecessarily doped to a high level to achieve a sufficient electricalconductivity to conduct charge away from the surface as it isintroduced, to maintain the surface voltage low. This prior approach isfully operable and works well in many circumstances. However, it reachesits limits where very high external electron fluxes are encounteredbecause the conventional pigment material cannot be doped tosufficiently high levels to achieve the required conductivity, whileretaining its desired color characteristics for thermal control. In thepresent approach, the excess charge is stored at the surface of theferroelectric pigment material, and later leaked to ground after theexternal electron flux has ceased.

FIG. 4 illustrates the relative permittivity ε_(r) of a ferroelectricpigment material having a ferroelectric/paraelectric transition as afunction of temperature T. At lower temperatures, where theferroelectric pigment material is in the ferroelectric state, therelative permittivity rapidly increases with increasing temperature to amaximum value at the ferroelectric Curie temperature T_(C). Above theCurie temperature, the ferroelectric pigment material becomesparaelectric, and the relative permittivity gradually falls with furtherincreasing temperature.

FIGS. 5 and 6 respectively illustrate the behavior of theferroelectric/paraelectric pigment material in its two states, andprovide a comparison with the prior, non-ferroelectric types of pigmentmaterials. When the ferroelectric pigment material is below the Curietemperature and thence in its ferroelectric state, its polarizationbehavior P (expressed in micro-coulombs per square centimeter) in anelectric field E (expressed in volts per meter) is illustrated in FIG.5. The electric field E is produced by the external electron flux. Froman initial state 32 with no applied electric field, an increasingelectric field causes the material to polarize along line 34, until amaximum polarization is reached at point 36. When the electric field isreduced as the external electron flux ends, the polarization rapidlyrelaxes along line 38 to a remnant polarization at point 40. Where thematerial is electrically conductive, there may be thereafter a gradualrelease of the remnant polarization with zero applied electric fieldalong line 42, possibly back to the initial state 32.

When the ferroelectric pigment material is above the Curie temperatureand thence in its paraelectric state, its polarization behavior P in theelectric field E is illustrated in FIG. 6. From an initial state 44, anincreasing electric field causes the polarization to increase along line46 to a maximum value 48. When the electric field is reduced as theexternal electron flux ceases, the polarization rapidly relaxes alongline 50 to the initial state 44. The ferroelectric state of FIG. 5 thusexhibits a hysteresis behavior in the polarization, but none is observedfor the paraelectric state.

FIG. 6 also illustrates the polarization behavior of a conventional,non-ferroelectric paint pigment material such as those used in the pastin paints for spacecraft. As the applied electric field increases alongline 52, there is only a small amount of polarization and P remainssubstantially zero. After reaching a maximum electric field and theelectric field is decreased along line 54, P relaxes toward zero.Consequently, only a small amount of electric charge is stored at thesurface of the coating material.

The ferroelectric pigment material of the present approach is preferablyoperated in the paraelectric state, with as large a permittivity aspossible so that the voltage V_(S) at the surface 30 of the coatinglayer 20 remains small. The paraelectric state is preferred because therelaxation of the polarization is rapid along line 50, and there is noremnant polarization as illustrated for the ferroelectric state alongline 42 in FIG. 5. The presence of a remnant polarization could resultin an indefinitely long storage of charge in the coating and anaccumulation of charge that could lead to dielectric breakdown of thecoating that could lead to a high-energy electrical discharge.Desirably, the dielectric permittivity of the ferroelectric pigmentmaterial is in the range of from about 200 to about 25,000. If it islower, the invention is still operable, but the voltage V_(S) at thesurface 30 becomes increasingly large with decreasing dielectricpermittivity, for a large electron flux.

Referring to FIG. 4, these conditions of paraelectric state and highdielectric permittivity are achieved when the ferroelectric pigmentmaterial is selected such that its Curie temperature T_(C) is slightlybelow the minimum surface operating temperature of the coating layer,T_(OP). The minimum surface operating temperature for the side of aspacecraft facing the sun is typically known from calculations ofthermal performance and/or from actual measurements of surfacetemperatures. For example, for a communications satellite in ageosynchronous orbit, the minimum surface operating temperature of awhite coating layer is typically about −70° C. (although in some casesit may be lower). The ferroelectric/paraelectric coating pigmentmaterial is therefore preferably selected so that its Curie temperatureis from about 10° C. to about 20° C. below the minimum operatingtemperature, or from about −90° C. to about −80° C. in this presentlypreferred case of a minimum surface operating temperature of about −70°C. These temperatures are presented for a typical case that is presentlypreferred and may vary for spacecraft in other orbits or with otherthermal properties, or if other surfaces of the spacecraft than thatfacing the sun are to be protected.

Although the preferred operational state of the ferroelectric pigment isabove the Curie temperature T_(C), and lying within the paraelectricphase state as shown by FIGS. 4 and 6, the pigment is also operablewithin the ferroelectric phase state. Preferably, the electricalhysteresis as shown by FIG. 5, and enclosed by lines 34, 38, and 42, issmall. This behavior is the case for many ferroelectric materialsoperating below the Curie temperature T_(C). Thus, the Curie temperatureT_(C) of the ferroelectric pigment material need not necessarily liebelow the operating temperature T_(OP) of the spacecraft as shown inFIG. 4, but it is preferred that T_(C) lies below T_(OP).

FIG. 7 summarizes a preferred method for preparing the particles 22, forpreparing the coating material used in the coating layer 20, and forcoating the substrate.

To prepare the ferroelectric pigment or particles 22, the components areprovided and mixed together, numeral 60. Any material exhibitingferroelectric/paraelectric behavior—a “ferroelectric” material—may beused, but design considerations aid in selecting a specificferroelectric/paraelectric material for use as the ferroelectricpigment. The ferroelectric/paraelectric material is preferably selectedso that its Curie temperature T_(C) is just below the spacecraft minimumsurface operating temperature T_(OP). The Curie temperature is known formany ferroelectric/paraelectric materials, and may be readily determinedfor other materials by finding the temperature associated with themaximum value in the dielectric permittivity as illustrated in FIG. 4 orby other techniques. The value of T_(OP) is known from measurements orcalculations of spacecraft surface conditions.

Another important property of the ferroelectric pigment material is itscolor, which largely determines its solar absorptance and thence itsthermal behavior. Materials with ferroelectric/paraelectric behavior maybe manufactured in a wide range of colors. The pigment color of mostinterest for use in a coating on a sun-facing surface is white, whichresults in a white coating with low solar absorptance. Otherferroelectric/paraelectric materials with other colors may be used inother applications. The ferroelectric/paraelectric material alsodesirably has a high index of refraction, which improves its hidingpower for use in a coating. Good hiding power results in the ability touse a thin coating layer, an important cost and weight consideration.

A presently most preferred ferroelectric material system exhibitingferroelectric/paraelectric properties isSrTiO₃—BaTiO₃—CaTiO₃—SrO—BaO—TiO₂—D, where D is a dopant such as La₂O₃,Nb₂O₅, Ga₂O₃, Gd₂O₃, or Y₂O₃, and mixtures thereof. This system offerssignificant potential for the tailoring of the properties of theferroelectric pigment to achieve particular characteristics of thecoating. The ratio Sr/Ba determines the Curie temperature. The calciumcontent determines the breadth in the peak (as in FIG. 4) in theferroelectric-paraelectric transition. Extra strontium, barium, andtitanium are used to balance the chemical stoichiometry duringprocessing. The lanthanum, niobium, gallium, gadolinium, and yttrium areused as dopants to make the material electrically leaky.

Within this system, a material of particular interest is lanthanum-dopedstrontium barium titanate, depicted as (Sr,Ba)TiO₃:La³⁺, which is whitein color and has an index of refraction of about 2.5. For the bariumtitanate form, BaTiO₃:La³⁺ with a lanthanum dopant concentration ofabout 1 weight percent La₂O₃, this material has a Curie temperature ofabout 125° C. measured in the undoped form. The maximum dielectricpermittivity, observed at the Curie temperature, is about 6000-7000.

Many other ferroelectric materials are known and may be used in relationto the present invention. Examples include BaTiO₃ mixed with MgSnO₃,SrTiO₃ mixed with CaTiO₃, CaSnO₃ mixed with CaO, CaZrO₃, CaSnO₃, andBi₂(SnO₃)₃. Relaxor ferroelectric material systems such asPb(Mg_(1/3)Nb_(2/3))O₃—PbTiO₃Ba(Zn_(1/3)Nb_(2/3))O₃ are additionalexamples of operable ferroelectrics.

The components of the particles are provided and mixed together, numeral60. In the presently preferred formulation, strontium titanate,strontium carbonate, barium carbonate, barium titanate, titanium oxide,titanium dioxide, and/or lanthanum oxide are used as starting materials,as appropriate to the selected composition. Thus, to prepare theferroelectric pigment composition, the appropriate amounts of thesestarting materials are finished and mixed together. A mixing medium,which later is removed, may be added to promote the mixing of thecomponents. Non-polar solvents such as hexane are the preferred mixingmedium, although other solvents such as water may also be used.Dispersants and surfactants may be used in the mixing medium, asrequired.

The components and the mixing medium are milled together to form amechanical mixture, numeral 62. After milling is complete, the mixingmedium is removed by evaporation, numeral 64.

The dried mixture is fired to chemically react the components togetherand annealed, numeral 66, at any operable temperature for the selectedferroelectric material. The firing, annealing, and subsequent coolingare performed in a nonoxidizing atmosphere, preferably an inertatmosphere such as helium or argon gas.

After cooling, the agglomerated mass resulting from the firing/annealingis lightly pulverized, as with a mortar and pestle, numeral 68. Theresulting particulate has a size range of from about 0.1 micrometers toabout 5.0 micrometers. The preparation of the particulate ferroelectricpigment is complete.

If the required ferroelectric pigment material is available in finalform, the steps 60-68 may be skipped and the ready-made startingmaterial may be used.

FIG. 7 provides a basic outline of a preferred synthesis technique forthe pyroelectric/ferroelectric pigment using the method of mixed oxides.However, other pigment synthesis techniques are operable and may be usedin certain circumstances such as those of chemical co-precipitationtechniques and hydrothermal techniques. For example, such techniquesmight be used to produce pigment particles of more uniform sizedistribution or higher purity pigments.

In addition to the described ferroelectric/paraelectric pigmentparticles 22 a, the material may contain active or inert secondaryparticles to modify the optical properties and/or the mechanicalproperties of the final material. Electrically conductive particles maybe introduced. Active secondary particles 22 b interact optically withincident energy, and include, for example, aluminum-doped zinc oxideparticles such as disclosed in U.S. Pat. No. 5,094,693 or the particlesdisclosed in U.S. Pat. No. 5,820,669, whose disclosures are incorporatedby reference. The active secondary particles 22 b such as thosedisclosed in U.S. Pat. No. 5,820,669 may be present in a greater amountthan the ferroelectric particles 22 a, for some applications. Suchactive secondary particles 22 b may be utilized to improve thelow-temperature electrical conductivity or optical properties forparticular applications. Inert secondary particles 22 c are thoseparticles which serve primarily as filler to increase the volumefraction of particulate material present without greatly modifying theoptical properties. The inert secondary particles 22 c may be added foreconomic reasons, as they are of lower cost than the ferroelectricpigment particles 22 a and active secondary particles 22 b. Inertsecondary particles 22 c can include, for example, barium sulfate, clay,or talc. In one example, the total particulate loading (total ofparticles 22 a, 22 b, and 22 c) could be 10 percent by volumeferroelectric/paraelectric pigment particles 22 a, 80 percent by volumeof the secondary particles 22 b of the '669 patent, and 10 percent byvolume inert particles 22 c.

The coating is prepared by providing the particulate material, preparedas described above or otherwise. A binder is provided, numeral 70, toadhere the particles together in the final product. The binder isselected to provide good coherence within the coating and good adherenceof the coating to the underlying substrate, with acceptable physicalproperties. The binder must withstand the conditions found in the spaceenvironment, such as temperature and radiation, and must also becompatible with the environment to which the coating is exposed, such aslow outgassing. The binder may be either an inorganic material or anorganic material.

A preferred inorganic binder for space applications is potassiumsilicate.

The preferred organic binder for space applications is cross-linked andpolymerized dimethyl silicone copolymer, which is flexible and resistantto degradation in ultraviolet (UV) light. This binder is disclosed ingreater detail in U.S. Pat. No. 5,589,274, whose disclosure isincorporated by reference. The silicone polymer exhibits a good degreeof deformability without cracking, both when the ferroelectric pigmentis present at moderate levels and when it is not present. Thisdeformability permits the final solid coating to deform during thebending of the substrate when a thin substrate is used, or to permit thefilm to deform. The deformability of the binder also improves theresistance of the coating to cracking as a result of impacts and thelike during service. Other flexible polymeric materials may be used forthe matrix, such as silicone-modified epoxy, polyurethane,poly(dimethyl-siloxane), poly(dimethylsiloxane-comethylphenylsiloxane),polyimide, and polyamide. However, experience has shown that thedimethyl silicone copolymer has the highest resistance to UWdegradation, and it is therefore preferred.

The binder is present in an operable amount. In a typical case, thebinder is present in an amount such that the ratio, by weight, of thetotal of all of the particulate to the binder is about 5:1 or less. Ifthe ratio is more than about 5:1, the critical pigment volumeconcentration (CPVC) may be exceeded and mechanical performance isreduced. Preferably, the ratio by weight of particles to binder is fromabout 2:1 to about 5:1.

The mixture of pigments 22 and binder 24 is ordinarily a solid, and acoating vehicle may be added to form a solution or a slurry that may beapplied using conventional coating techniques, numeral 72. One preferredcoating vehicle for an inorganic-binder system is water, which does nothave adverse environmental impacts when later evaporated. Organiccoating vehicles such as naphtha or xylene may also be used,particularly for organic-binder systems. The amount of the coatingvehicle is selected to provide a consistency that will permitapplication of the coating by the desired approach. For example,application by spraying requires the use of more of the coating vehiclethan application by brush or roller.

The coating may instead be applied by a technique where no vehicle isused, and in that case the step 72 is omitted.

The particles, binder, and optional coating vehicle are mixed togetherand milled together, numeral 74, to form a liquid coating formulation inwhich the particles do not rapidly separate. There may be someseparation over extended periods of time, but the coating is normallystirred or agitated just before or at the time of application.

In an alternative approach to the depicted order of the steps 70, 72,and 74, the particles may be milled in water, such as de-ionized water.The binder is thereafter furnished and added to the milled particles andwater. These components are further mixed to disperse the particlesthroughout the binder.

The preparation of the coating material is complete.

The coating is used by providing the substrate 26 to be coated (such asthe spacecraft external surface), numeral 76, and cleaning thesubstrate, numeral 78. There is no known limitation on the type ofsubstrate. The surface of the substrate is cleaned by any operabletechnique, such as washing and scouring in a detergent solution, rinsingin tap water, rinsing in de-ionized water, and drying in air.

The coating is applied to the surface of the substrate, numeral 80. Atthe outset of the application, the surface of the substrate may beprimed to improve the adhesion of the coating. Priming is preferred forapplication of the coating containing an inorganic binder to metallicsurfaces such as aluminum. Preferably, the priming, if used, isaccomplished by rubbing a small amount of the coating into the surfaceusing a clean cloth, to achieve good contact to the surface.

The coating layer is thereafter applied by any operable technique, withspraying being preferred. As indicated earlier, the amount of coatingvehicle present in the coating is selected to permit application by thepreferred approach. At this point, the coating is a thin film of aliquid material containing the pigment particles.

The coating may also be applied by a plasma spray technique or the likewherein the mixture of pigment and binder is supplied to a heated regionsuch as a plasma and directed toward the substrate. The plasma-heatedmixture of pigment and binder strikes the substrate and solidifiesthereon.

The coating is dried as necessary to leave a thin film of a solidmaterial, numeral 82. In the case of an inorganic binder, drying ispreferably accomplished at ambient temperature with a 50 percent orgreater humidity and for a time of 14 days. Drying removes the coatingvehicle by evaporation. Additionally, the drying step may accomplish adegree of curing of any curable components, as where a curable inorganicor organic binder is used. After drying and/or curing, the coating layeris preferably from about 0.001 inch to about 0.004 inch thick, mostpreferably about 0.002 inch thick. This thickness is somewhat less thanthat of conventional protective coatings, which are usually from about0.003 to about 0.006 inch thick, resulting in a materials cost andweight savings.

The coating may be applied as a single coat, or as multiple coats whichare dried between coats. Even where multiple coats are applied, thecoating is still a “single layer” coating, because its composition issubstantially homogeneous throughout all of the coats and between thecoats. This single-layer coating is to be contrasted with multi-layercoatings found in some of the prior art, wherein the various layers areintentionally of greatly different compositions and performances.

The coating is complete.

The coating of the invention may be used in any thermal controlapplication. Most preferably, it is used as a coating on a spacecraft,such as a spacecraft 90 illustrated in FIG. 8. The spacecraft 90, heredepicted as a communications satellite that is positioned ingeosynchronous orbit when in service, has a body 92 with solar panels 94mounted either on the body 92 or on wings 96 that extend outwardly fromthe body 92, or both. The body 92 and wings 96 have a skin 98 which maybe made of a metal, a nonmetal, or a composite material, and which maybe supported by an underlying skeletal structure. The external surfaces25 of at least some of those outwardly facing portions of the skin 98 ofthe body 92 and/or the wings 96 which are not solar panels are coveredwith the layer 20 of the coating of the invention, as described above.Typically, the coating in a white-coating form is applied to thesun-facing surfaces, as shown. The skin 98 of the spacecraft therebyserves as the substrate 26 to which the coating layer 20 is applied. Thecoating layer 20 provides the covered portions with passive thermalcontrol and protection against electron fluxes that may cause electricaldamage. The coating is sufficiently durable and stable in its propertiesfor use on extended missions.

When the ferroelectric pigment is made operable in the ferroelectricmode, the optical characteristics of the coating may be made to changefrom that of an “adaptive” passive mode of operation to that of an“active” mode of operation. Some ferroelectric materials, such as PLZT(lead lanthanum zirconate titanates), are available in a bulk solid,transparent form, with a band gap of more than about 3 electron volts.The light transmission characteristics of such materials, such as theirbirefringence, may be changed through the application of an externalelectric field by changing the polarization state of the material. Thelight-reflecting characteristics of the coating layer 20 may be alteredby application of an external electrical field to such a coatingmaterial. The coating material then becomes an “active” thermal controlcoating, having the ability to change solar absorptance and reflectancecharacteristics upon the application of an external electrical field.This ability to change optical characteristics is useful when, forexample, the coating layer is exposed to cycles of sunlight and darknesswherein it is desirable to decrease or increase the thermal heattransfer to the spacecraft within the cycle. The optical characteristicsmay also be adjusted to compensate for the decreased or increased heatrequirements of the spacecraft made by operation of the internalcommunications electronics. These capabilities are not available withconventional spacecraft paints.

FIG. 9 illustrates an active control approach for the coating layer 20.In FIG. 9, the coating layer 20 is represented by its equivalent circuitof FIG. 3, whose description is incorporated. The coating layer 20 isconnected to one terminal of a variable-bias voltage supply 100, whoseother terminal is connected to a reference voltage such as thespacecraft body 102 that is nominally at ground potential. Thetemperature of the spacecraft body 102 or its skin is measured by asensor 104, such as a thermocouple. The output signal of the sensor 104is provided to an onboard computer 106. The onboard computer 106observes changes of the temperature from the desired value, and adjuststhe voltage of the variable-bias voltage supply 100 accordingly tomaintain a “setpoint” value of temperature. The bias voltage on thecoating layer 20 is thereby established. The variable-bias voltagesupply 100, the spacecraft body 102, the sensor 104, and the computer106 thus form a feedback loop, indicated by numeral 108, for autonomouscontrol of the temperature of the spacecraft through active control ofthe coating layer 20. The temperature may also be controlled from anexternal source through adjustments to the “setpoint” temperaturecontrol of the computer 106. FIG. 9 illustrates a preferred approach forexternal control, a command uplink from a ground control station 110.

Although particular embodiments of the invention have been described indetail for purposes of illustration, various modifications andenhancements may be made without departing from the spirit and scope ofthe invention. Accordingly, the invention is not to be limited except asby the appended claims.

What is claimed is:
 1. A spacecraft protected by a coating, comprising:a spacecraft having an external surface, and a coating on the externalsurface of the spacecraft, the coating including a binder, and aplurality of pyroelectric pigment particles bound together by thebinder, each pyroelectric pigment particle comprising a pyroelectricpigment material.
 2. The spacecraft protected by the coating of claim 1,wherein the spacecraft is a communications satellite.
 3. The spacecraftprotected by the coating of claim 1, wherein the binder is an inorganicmaterial.
 4. The spacecraft protected by the coating of claim 1, whereinthe binder is an organic material.
 5. The spacecraft protected by thecoating of claim 1, wherein the pyroelectric pigment material is whitein color.
 6. The spacecraft protected by the coating of claim 1, whereinthe coating has a thickness of from about 0.001 inch to about 0.004inch.
 7. The spacecraft protected by the coating of claim 1, wherein theratio by weight of pyroelectric pigment particles to binder is about 5:1or less.
 8. The spacecraft protected by the coating of claim 1, whereinthe coating further includes a plurality of active secondary particles.9. The spacecraft protected by the coating of claim 1, wherein thecoating further includes a plurality of inert particles.
 10. Thespacecraft protected by the coating of claim 1, wherein the pyroelectricpigment material is a ferroelectric material having aferroelectric/paraelectric transition.
 11. The spacecraft protected bythe coating of claim 10, wherein the ferroelectric material has adielectric permittivity of from about 200 to about 25,000.
 12. Thespacecraft protected by the coating of claim 10, wherein theferroelectric material has an electronic band gap exceeding about 2.5electron volts.
 13. The spacecraft protected by the coating of claim 10,wherein the external surface has a minimum operating temperature ofT_(OP), and the ferroelectric pigment material has a ferroelectric Curietemperature of less than T_(OP).
 14. The spacecraft protected by acoating of claim 1, wherein the spacecraft further includes a biasvoltage supply in communication with the coating.
 15. The spacecraftprotected by a coating of claim 14, wherein the spacecraft furtherincludes a feedback controller for the bias voltage supply.
 16. Aspacecraft protected by a coating, comprising: a spacecraft having anexternal surface, and a coating on the external surface of thespacecraft, the coating including a binder, and a plurality ofpyroelectric pigment particles bound together by the binder, eachpyroelectric pigment particle comprising a pyroelectric pigment materialhaving a dielectric permittivity of from about 200 to about 25,000 andan electronic band gap exceeding about 2.5 electron volts.
 17. Thespacecraft protected by the coating of claim 16, wherein the coatingfurther includes a plurality of active secondary particles.
 18. Thespacecraft protected by the coating of claim 16, wherein the coatingfurther includes a plurality of inert particles.
 19. The spacecraftprotected by a coating of claim 16, wherein the spacecraft furtherincludes a bias voltage supply in communication with the coating. 20.The spacecraft protected by a coating of claim 19, wherein thespacecraft further includes a feedback controller for the bias voltagesupply.
 21. A spacecraft protected by a coating, comprising: aspacecraft having an external surface, and a coating on the externalsurface of the spacecraft, the coating including a binder, and aplurality of pigment particles bound together by the binder, eachpigment particle comprising a pigment material which stores electroniccharge when exposed to an electron flux, and which thereafter releasesthe stored electronic charge over a period of time.
 22. The spacecraftprotected by the coating of claim 21, wherein the coating furtherincludes a plurality of active secondary particles.
 23. The spacecraftprotected by a coating of claim 21, wherein the coating further includesa plurality of inert particles.
 24. The spacecraft protected by acoating of claim 21, wherein the spacecraft further includes a biasvoltage supply in communication with the coating.
 25. The spacecraftprotected by a coating of claim 24, wherein the spacecraft furtherincludes a feedback controller for the bias voltage supply.
 26. Acoating material, comprising: a binder; a plurality of pyroelectricpigment particles, each pyroelectric pigment particle comprising apyroelectric pigment material having a ferroelectric/paraelectrictransition; and a plurality of active secondary particles, the activesecondary particles being electrically conductive and non-ferroelectric,wherein the plurality of pyroelectric pigment particles and theplurality of active secondary particles are bound together by thebinder.
 27. A coating material, comprising: a binder; and a plurality ofdoped pyroelectric pigment particles, the particles being doped to asufficient extent that the electrical surface resistivity of the coatingmaterial is less than or equal to about 10¹⁰ ohms per square at roomtemperature.